Medical procedures often require locating and treating target areas within a patient. Radiation therapy and many surgical procedures require locating the target with a high degree of precision to limit collateral damage to healthy tissue around the target. It is particularly important to know or estimate the precise location of the target in radiation oncology because it is desirable to limit the exposure of adjacent body parts to the radiation. In applications for treating prostate cancer, for example, the colon, bladder or other body part of the patient adjacent to the prostate is desirably not impinged by the high-intensity radiation beam. Surgical applications, such as breast surgery and other procedures involving soft tissue, also require knowing the precise location of a target because a lesion is not necessarily fixed relative to external landmarks on the patient.
Many imaging systems have been used to locate areas or particular targets within a body before performing radiation oncology or surgical procedures. Although x-ray, Magnetic Resonance Imaging (MRI), CT, and other imaging techniques are useful to locate targets within the body at the pre-operative stage of a procedure, they are often not suitable or difficult to use in real time during surgery or radiation therapy. For example, the location of a lesion in soft tissue or an organ within the patient's body may shift relative to external landmarks on the patient between the pre-operative imaging procedure and the actual radiation or surgical procedure. Additionally, when imaging systems are used during a radiation or surgical procedure, they may not provide sufficiently accurate measurements of the location of the lesions and they may interfere with the radiation or surgical procedure. Therefore, imaging techniques by themselves are not suitable for accurately identifying the actual location of a target for many medical applications.
Another technique to locate a target in a patient is to implant a marker relative to the target. For example, implantable markers that generate a signal have been proposed for use to locate a selected target in a patient in radiation oncology procedures. U.S. Pat. No. 6,385,482 B1 issued to Boksberger et al. discloses a device having an implanted emitter unit SE located inside or as close as possible to a target object T and a plurality of receiver units S11, S12, S21 and S22 that are located outside of the patient. Boksberger discloses determining the location of the target object T by energizing the emitter unit SE using generator GE and sensing the signal from the emitter unit SE with the receiver units S11-S22. Boksberger discloses and claims that the receiver units S11-S22 are configured to determine the gradient of the magnetic field generated by the emitter unit SE. Boksberger discloses emitter units SE that are energized using a wired connection to the external generator GE. Boksberger also indicates that it is conceivable to use an emitter unit SE that is energized by a battery or excited by an electromagnetic field generated by the external generator GE. The wired device disclosed in Boksberger, however, may not be suitable for use in radiation oncology and many surgical procedures because it is impractical to leave a wired marker implanted in a patient for the period of time of such procedures (e.g., five to forty days). Moreover, Boksberger does not disclose or suggest anything with respect to providing an implantable emitter unit SE that is compatible for use in magnetic resonance imaging devices after being implanted in a patient.
Another technique to locate a target in a patient is to implant passive, gold fiducials in or near the target site. The positions of the gold fiducials are determined periodically using radiation. Although gold fiducials are useful for localizing a target within a patient, these systems do not provide sufficiently accurate real time measurements of the target site location during radiation oncology procedures.
Other types of tags or markers with resonating magnetic circuits have been developed. These markers have been used to tag sponges and other items used during surgery or locate the general location of feeding tubes or other instruments in other procedures. One significant challenge of miniature, wireless markers is to provide a sufficiently strong signal to be accurately detected by sensors outside of the body.